The inventors have newly developed a corrosion resistant alloy having improved workability and pickling performance which comprises in % by weight up to 0.05% of C, 10.00 to 18.00% of Cr, up to 1.00% of Si, up to 1.00% of Mn, more than 0.040% but not more than 0.150% of P, up to 0.050% of S, up to 0.60% of Ni and 0.005 to 0.50% of sol. Al, and optionally one or both of up to 1.00% of Cu and up to 1.00% of Mo, and further optionally one or both of up to 0.50% of Ti and up to 0.50% of Nb in an amount of up to 0.50% in total, the balance being Fe and unavoidable impurities.
The invention provides a process for the production of a cold rolled strip of the above-mentioned novel alloy, which process permits the production of a product having a further enhanced workability, thereby to provide an inexpensive strip of the corrosion resistant alloy steel having an excellent workability.
Whem compared with existing ferritic stainless steels, a variety of corrosion resistant materials, our novel alloy is prescribed so that it contains a higher level of P(more than 0.040% but not more than 0.150% P) than that of the existing ferritic stainless steels, although our alloy has a corrosion resistance comparable to that of the existing ferritic stainless steels. Accordingly, it is possible to prepare our alloy by directly feeding pig iron from a blast furnace to a converter without the necessity of a special treatment for removing P from such pig iron and adding suitable subsidiary materials such as Fe-Cr alloys to the converter. In addition, the pickling performance of hot rolled strips is for superior with our alloy than with the existing ferritic stainless steels. Accordingly, enhancement of the productivity and great reduction in the manufacturing costs may be enjoyed with our new alloy, enabling the provision of inexpensive strips of a corrosion resistant alloy steel.
Thus, strips of the new alloy can be a substitute for the existing strips of ferritic stainless steels. Moreover, they may be used in such applications where plated or coated strips of ordinary steels cheaper than stainless steels have heretofore been used although they are not satisfactory regarding corrosion resistance.
In many cases steel strips are used after having been formed into shapes, e.g., by pressing, and therefore, the workability of steel strips is very important. With the novel alloy discussed above a further improvement of the workability is strongly desired.
Cold rolled strips or sheets of ferritic stainless steels are basically produced by a process including the steps of hot rolling a slab to a hot rolled strip (or sheet), optionally annealing the hot rolled strip, descaling the strip by pickling, cold rolling the strip and subjecting the cold rolled strip to a final or finish annealing. The cold rolling may be carried out in one stage or in multiple stages. In the latter case, an intermediate annealing may be carried between any adjacent stages of cold rolling.
As to annealing, there are two types of, one is a continuous annealing while the other is box annealing. In a continuous annealing, a running steel strip is caused to pass through an annealing furnace maintained at a predetermined annealing temperature. Usually the material to be annealed is rapidly heated at a rate of heating of at leasst 200.degree. C./min. and allowed to cool in air. Accordingly, the period of time during which the material is held at the annealing temperature is very short.
On the other hand in a box annealing a stationary steel strip in the form of a coil is annealed. Usually the material is slowly heated at a rate of heating of 300.degree. C./hr or below. A period of time during which the material is held at the annealing temperature is much longer than that in a continuous annealing, and the annealed material is slowly cooled, e.g., by being allowed to stand in the annealing furnace.
While an anneal of a hot rolled strip of ferritic stainless steel may be carried out either in a box annealing furnace at a slow rate of heating or in a continuous anneal furnace at a fast rate of heating, a final annealing in the case of one stage cold rolling as well as any intermediate annealing or annealings and a final annealing in the case of multiple stage cold rolling have been normally carried out in a continuous annealing furnace at a fast rate of heating.